Well, I was told this TCU (mine and Stringbag ones, I dont know about yours) talks with handset using a bluetooth connection.
Problem is that they make use of proprietary BT profiles instead of the standard ones.
A did some tests in the past, when mine was still working, with several phones (Samsung SGH-D500, Samsung SGH-Z560, Nokia N85 8Gb, Imate JasJar and Motorola V3X) and none of them did even recognize there was another BT a couple meters away.
Just note that if you turn handset off and then back on while ignition is on, handset display will show "reconnecting xxxx" (where xxxx is BT pairing code) and then it succesfully connects in a few seconds... no need to start a pairing procedure.
This behaviour makes me think that BT discovery mode should be enabled by default, so why other phones don't even see there is another BT around..?!?